The disclosure relates in general to Magnetoresistive Random Access Memories (MRAMs), and more particularly to magnetic tunnel junction (MTJ) MRAM arrays and a specific architecture for connecting the arrays.
Magnetic random access memory (MRAM) cells are often based on a magnetic tunnel junction (MTJ) cell. Basically, an MTJ configuration can be made up of three basic layers, a “free” ferromagnetic layer, an insulating tunneling barrier, and a “pinned” ferromagnetic layer. In the free layer, the magnetization moments are free to rotate under an external magnetic field, but the magnetic moments in the “pinned” layer cannot. The pinned layer can be composed of a ferromagnetic layer and/or an anti-ferromagnetic layer which “pins” the magnetic moments in the ferromagnetic layer. A very thin insulation layer forms the tunneling barrier between the pinned and free magnetic layers. In order to sense states in the MTJ configuration, a constant current can be applied through the cell. As the magneto-resistance varies according to the state stored in the cell, the voltage can be sensed over the memory cell. To write or change the state in the memory cell, an external magnetic field can be applied that is sufficient to completely switch the direction of the magnetic moments of the free magnetic layers.
MTJ configurations often employ the Tunneling Magneto-Resistance (TMR) effect, which allows magnetic moments to quickly switch the directions in the magnetic layer by an application of an external magnetic field. Magneto-resistance (MR) is a measure of the ease with which electrons may flow through the free layer, tunneling barrier, and the pinned layer. A minimum MR occurs in an MTJ configuration when the magnetic moments in both magnetic layers have the same direction or are “parallel”. A maximum MR occurs when the magnetic moments of both magnetic layers are in opposite directions or are “anti-parallel.”
FIG. 1A is a schematic perspective view illustrating a conventional MTJ cell of an MRAM device. FIG. 1B is a schematic perspective view illustrative of read out operation of the MTJ cell of FIG. 1A. FIG. 1C is a plane view illustrative of respective magnetization states depending on stored data of the MTJ cell of FIG. 1A.
The single memory cell comprises: a first metal layer 11; a pinned layer 12; a tunnel barrier layer 13; a free layer 14; and a second metal layer 12. The MTJ cell comprises: the pinned layer 12; the dielectric layer 13; and the free layer 14. The tunnel barrier layer 13 is sandwiched between the pinned layer 12 and the free layer 14. The pinned layer 12 is in contact with the first metal layer 11. The free layer 14 is in contact with the second metal layer 12. The pinned layer 12 and the free layer 14 are made of ferromagnetic materials. The dielectric layer 13 is made of an insulating material. The pinned layer 12 has a fixed magnetization direction. The dielectric layer 13 has a thickness of about 1.5 nanometers. The free layer 14 has a thickness of about 20 nanometers. The free layer 14 has a freely changeable magnetization direction.
The magnetization direction of the free layer 14 indicates stored data. The free layer 14 serves as a data storage layer. The first metal layer 11 and the second metal layer 15 extend in directions perpendicular to each other. The MTJ cell is positioned at a crossing point between the first metal layer 11 and the second metal layer 15. In FIG. 1B, a current 16 flows from the first metal layer 11 to the second metal layer 15 through the pinned layer 12, the dielectric layer 13, and the free layer 14. The MTJ cell is capable of storing binary digit data “0” and “1”. If the magnetization directions of the pinned layer 12 and the free layer 14 are parallel to each other, then this means that the MTJ cell stores a first binary digit, for example, “0”. If the magnetization directions of the pinned layer 12 and the free layer 14 are not parallel, then this means that the MTJ cell stores a second binary digit, for example, “1”. The magnetization direction of the free layer 14 changes depending on an externally applied magnetic field.
An electrical resistance of the dielectric layer 13 varies by about 10-60% due to the tunneling magnetoresistance effect between in a first state where the magnetization directions of the pinned layer 12 and the free layer 14 are parallel to each other and a second state where the magnetization directions of the pinned layer 12 and the free layer 14 are not parallel. A predetermined potential difference or a predetermined voltage is applied to the first and second metal layers 11 and 15 to apply a tunneling current from the pinned layer 12 to the free layer 14 through the dielectric layer 13. This tunneling current varies depending on the variable electrical resistance of the dielectric layer 13 due to the tunneling magnetoresistance effect. The data can be fetched from the MTJ cell by detecting the variation in the tunneling current.
FIG. 2A is a fragmentary schematic perspective view illustrative of an array of MTJ cells of the MRAM of FIG. 1A. FIG. 2B is a fragmentary schematic perspective view illustrative of the array of the MTJ cells during the operation shown in FIG. 2A.
The first metal layers 11 extend in parallel to each other in a first direction. The second metal layers 15 extend in parallel to each other in a second direction, which is perpendicular to the first direction. The single first metal layer 11 and the single second metal layer 15 have a single crossing point, where a single MTJ cell “C” is provided. The plural first metal layers 11 and the plural second metal layers 15 have an array of crossing points where plural MTJ cells “C” are provided. The first metal layers 11 serve as word lines. The second metal layers 15 serve as bit lines. One of the plural MTJ cells “C” is selected by selecting one of the word lines and one of the bit lines, for read or write operations to the selected MTJ cell “C”.
The MRAM has the array of the MTJ cells, each of which comprises the tunneling magnetoresistance element utilizing the tunneling magnetoresistance effect, wherein the tunneling magnetoresistance element includes an insulating thin film sandwiched between the two or more ferromagnetic thin films. The tunneling magnetoresistance element is switched between a first state, in which the magnetization directions of the two ferromagnetic thin films are parallel to each other, and a second state, in which the magnetization directions of the two ferromagnetic thin films are anti-parallel. The resistance of the insulating film, which the tunneling current senses, is different for the first and second states. These two states correspond to binary digits, for example, the first state corresponds to the data “0”, and the second state corresponds to the data “1”.
The write operation is accomplished as follows. One of the word lines 11 and one of the bit lines 15 are selected. A first write current Isw is applied to the selected word line 11s. A first magnetic field Msw is generated around the selected word line 11s. The first write current Isw has a predetermined current value and a predetermined direction. A second write current Isb is applied to the selected bit line 15s. The second write current Isb has a predetermined current value and a predetermined direction. A second magnetic field Msb is generated around the selected bit line 15s. As a result, a superimposed magnetic field of both the first and second magnetic field Msw and Msb is applied to the crossing point of the selected word line 11s and the selected bit line 15s. The selected MTJ cell “Cs” is positioned at the crossing point of the selected word line 11s and the selected bit line 15s, for which reason the selected MTJ cell “Cs” is applied with the superimposed magnetic field. The free layer of the selected MTJ cell “Cs” is also applied with the superimposed magnetic field, whereby magnetic domains of the free layer become ordered in a first direction, for example, in a direction parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ cell “Cs” stores a binary digit data “0”.
Any first write current Isw or second write current Isb changes its current direction to an opposite direction, whereby the direction of the magnetic field is inverted, and the direction of the superimposed magnetic field is changed by approximately 90 degrees. As a result, the magnetic domains of the free layer become ordered in a second direction, for example, in a direction anti-parallel to the magnetization direction of the pinned layer. As a result, the selected MTJ cell “Cs” stores another binary digit “1”.
The read operation is accomplished as follows. One of the word lines 11 and one of the bit lines 15 are selected. A potential difference is applied between the selected word line 11s and the selected bit line 15s for measuring a current value to detect a resistance value of the selected memory cell “Cs” to the tunneling current. Namely, a predetermined potential difference or a predetermined voltage is applied to the selected word line 11s and the selected bit line 15s to provide a tunneling current from the pinned layer through the insulating layer to the free layer of the selected memory cell “Cs”. This tunneling current varies depending on the variable electrical resistance of the insulating layer due to the tunneling magnetoresistance effect. The binary digit data can be detected from the selected memory cell “Cs” by detecting the variation in the tunneling current.
FIG. 3 is a diagram illustrative of a conventional array structure of the MTJ cells in the MRAM. An array 21 includes 2m word lines (W1, W2, W3, - - - Wm, Wm+1, - - - , W2m) and 2n bit lines (B1, B2, B3, - - - Bn, Bn+1, - - - , B2n) as well as 2 m×2n MTJ cells (C11, C12, - - - , C2m2n) that are positioned at crossing points of the word and bit lines. A word line Wig and a bit line Bj are selected to select a MTJ cell Cij positioned at the crossing point between the selected word line Wi and the selected bit line Bj for read or write operation as described above.
As mentioned above, the first and second write current are provided to generate the first and second magnetic fields around the selected conductive lines, respectively. For reducing the current density, a ferromagnetic layer adjacent to the conductive line would dramatically enhance the magnetic field generated by the conductive line. For linear response in the magnetic field by the conductive lines, the hard axis of the ferromagnetic layer is parallel to the magnetic field. Due to the enhancement in magnetic field by the ferromagnetic layer, less power will be consumed during a writing process. In FIG. 4, for linear response and zero remnant magnetization of the ferromagnetic layer, the easy axis EA of the ferromagnetic layer 32 would be set parallel to the current direction Id along the conductive line 31. Here, the easy axis EA of the ferromagnetic layer 32 is pinned by the anti-ferromagnetic layer 33 and substantially perpendicular to the circular magnetic field Hd generated by the current Id. Therefore, the ferromagnetic layer 32 inserted adjacent to the conductive line 31 will force the magnetization of the ferromagnetic layer 32 parallel to the current direction. Here, the ferromagnetic layer 32 and the anti-ferromagnetic layer 33 comprise a ferromagnetic cladding layer.
U.S. Patent Application 20020182557 discloses a method to heat-treat (or annealing process) a substrate provided with thin magnetic layers in a magnetic field to magnetize the pinned layer in one direction. Typically, an oriented magnetic field of 0.5 T (tesla) or more must typically be applied, and an oriented magnetic field of more than 1.0 T is often necessary depending on the materials of the pinned layer. To apply an oriented magnetic field to wafer substrates, a vacuum heat-treating furnace has conventionally been used. This vacuum heat-treating furnace comprises a magnetic field-generating coil equipped with a cooling pipe, a high-frequency coil disposed inside the coil, and a vacuum container for holding a plurality of wafer substrates disposed inside the high-frequency coil.
However, due to the substantially orthogonal write line structure, the ferromagnetic cladding layers, which respectively clad the word line and the bit line, are substantially perpendicular to each other. Here, the easy axes of the substantially perpendicular word line and bit line are also substantially perpendicular. Thus, the ferromagnetic cladding layers have to be exchange-biased with anti-ferromagnets exhibiting different anneal temperatures to form the ferromagnetic cladding layers of the bit line and the word line with perpendicular easy axes.
In addition, a complicated annealing process with at least two annealing steps in mutually perpendicular annealing processing-fields is typically necessary. The obvious disadvantage is that one ferromagnetic cladding layer in which anisotropy is set in the first annealing process is affected by the following second annealing process.